1. Field of the Invention
The present invention relates to an investment for dental casting, which is used as the casting material in the preparation of dental metal prostheses such as crowns, inlays and bridges by the precise casting process.
2. Statement of the Prior Art
Hitherto, dental metal prostheses have been made by the precise lost-wax casting process which assures excellent dimensional accuracy. Investments used as the casting materials in this case are generally broken down into gypsum base ones used with alloys having a relatively low melting point such as gold alloys, silver alloys and gold-silver-palladium alloys and phosphate base ones used with alloys having a relatively high melting point such as Ni-Cr base alloys and noble metal base alloys for porcelain fused materials. However, such investments have offered the following problems.
(1) The conventional investments make up for the casting shrinkage or contraction of metals by a combination of solidification expansion (including hygroscopic expansion) with thermal expansion. However, a part of the solidification expansion takes place unevenly enough to deform the wax pattern, resulting in the deformation of the resulting casting.
(2) The thermal expansion of quartz or cristobalite used as the refractory in the conventional investments is dependent upon heating temperature and is reversible. Thus, when a certain time elapses before the investments heated in an electrical furnace are cast by means of a casting machine, the investments are cooled down to a temperature much lower than 700.degree. C. at which the burning-out of wax takes place, until they are actually cast. In the meantime, the amount of thermal expansion tends to decrease. Especially when the mould is cooled down to about 300.degree. to 400.degree. C. following the burning-out of wax at 700.degree. C., as is the case with the casting of alloys having a melting point of about 500.degree. to 700.degree. C. such as silver alloys or silver-indium alloys, castings having suitable fitting accuracy are not obtained due to insufficient thermal expansion.
(3) A large amount of cristobalite is incorporated into the conventional investments so as to obtain thermal expansion. Since such thermal expansion occurs rapidly in the phase transformation temperature region, however, the moulds used become susceptible to cracking. As a result, cracking often occurs in the castings. This becomes marked especially when the rate of heating of the moulds is high in the phase transformation temperture region.
(4) For the purpose of bettering the reproducibility of the surface state of the wax pattern, it is required that the refractory particles be fined down to improve the heat resistance and surface of the castings. However, this gives rise to a decrease in the air permeability of the investments, which often leads to casting defects such as casting insufficiency. The flowability of the investments also drops in a slurried state so that difficulty is experienced in the investing manipulation of the wax pattern. For these reasons, it is impossible to fine the refractory particles down to lower than a certain particle size.
(5) Hemi-hydrate gypsum is used as the binder, when casting alloys having a relatively high melting point such as Ni-Cr base alloys or noble metal base alloys for porcelain fused materials. Since such hemi-hydrate gypsum tends to decompose thermally and be deposited to the casting surface, however, it is required to remove the deposited hemi-hydrate gypsum by means of a sandblaster, etc.
(6) In order to prevent surface roughening of the investments due to rapid evaporation of the in-mould moisture content after the investment of the wax pattern, the moulds have to be dried at 100.degree. C. or lower. With the conventional investments, however, much time is required for drying, since the rate of temperature increase in the moulds is low due to their poor thermal conductivity.
In view of the foregoing, the present inventors have already proposed, in Japanese patent application No. 59-138942, (U.S. Pat. No. 4,604,142) an investment for dental casting obtained by adding raw starch (original starch) with or without soluble starch to a mixture of hemi-hydrate gypsum with quartz and/or cristobalite. As disclosed therein, such an investment expands uniformly through thermal expansion alone to make up for the casting contraction of metals, allows the refractory particles to be divided finely without any decrease in the air permeability thereof, and shows suitable flowability in a slurried state.
It is admitted that the investment disclosed in Japanese patent application No. 59-138942 (U.S. Pat. No. 4,604,142) expand uniformly only by way of the thermal expansion involving the thermal expansion of raw starch (original starch) and the thermal expansion of quartz and cristobalite by phase transformation so as to compensate for the casting contraction of metals, allow refractory particles to be fined down without any decrease in the air permeability thereof, and show suitable flowability in a slurried state. However, there is still left much to be desired in view of:
(1) Insufficiency in the amount of thermal expansion, when cooling the investment in the casting of silver alloys, for instance.
(2) Occurrence of cracks in the investment, when heating it for the purpose of burning out wax.
(3) Deposition of the investment due to the casting surface, when casting an alloy having a high melting point. and,
(4) Drying of the mould over an extended period of time.
The insufficiency in the amount of thermal expansion of the prior art investment and the cracking therein are considered to be due to the fact that the prior art investment (Japanese patent application No. 59-138942) (U.S. Pat. No. 4,604,142) is thermally expanded through a combination of the expansion of raw starch (original starch) with the expansion-by-phase transformation of quartz or cristobalite. In Japanese patent application No. 59-138942, (U.S. Pat. No. 4,604,142) no attention is paid to the deposition of the investment onto the casting surface or the prolonged drying of the mould used.